[mythtv-users] Question re: available SATA ports and linux software RAID

[Alex Butcher]

On Thu, 7 Apr 2011, Bobby Gill wrote:

> Going to be setting up a server soon, am pretty settled on RAID5 linux
> software RAID. I currently have an ASUS P5K-E mobo that has 6 SATA slots + 2
> eSATA slots. I would be using 1 drive for my Arch OS, so that leaves 7
> available slots for my array.

RAID is for availability, rather than backup, so unless you like the idea of
doing unscheduled OS/MythTV reinstalls/restores, then it might be a good
idea to put your OS on more than one drive too.

Personally, my view is that with the price of storage as it is, parity RAID
is only useful when one needs lots of cheap storage and the data stored on
it is mostly quiescent; archives and the like. Otherwise, I prefer either
RAID1 or RAID10, depending on budget. Mine is probably a minority opinion,
though.

> My question is, what if I would wish to have a 8/10/12 drive array either
> right off the bat or in the future? Would getting one of these work:
> http://www.newegg.com/Product/Product.aspx?Item=16-101-358

I've not use SAS controllers before, but you should be able to connect SATA
drives to them. I don't know how well that'll work, though. As another
poster mentioned, you'll also need some kind of break-out cable/board.

> That would add 8 SATA slots. I am just trying to confirm that adding that
> card (or one like it) would do the trick of adding SATA slots and could be
> used as any other SATA slots for linux software RAID, so I wouldn't have to
> worry about not being able to physically add drives to my array in the
> future if desired.
> 
> (I was considering the HighPoint 2300 PCIe controller card but after
> communicating with their web support, they confirmed to me that multiple
> controller cards of theirs (any?) cannot be used in conjunction to form an
> array (one card = 4 slots so I couldn't buy 2 cards and build an 8 drive
> array on the same mobo).)

If you're using Linux software RAID (md), then you don't need a fancy RAID
controller, and the RAID capabilities of the card have no bearing on what
you can or can't do (as long as it can present all attached drives as
individual block devices). For this reason, it's generally not worth paying
extra for hardware RAID controllers unless you plan to use the facilities
that they provide.

HOWEVER... many of the chipsets used on cheap SATA controllers are buggy:

<http://codemonkey.org.uk/2009/01/20/sata-disasters-silicon-image-3114/>
<http://marc.info/?t=123089876500001&r=1&w=2>
<https://bugs.launchpad.net/ubuntu/+source/linux/+bug/263160>

The cheapest I found that seemed to be OK was The Promise SATA300 TX4. Note
that this is sold as a RAID card, but is actually only software RAID (aka
fakeraid); the controller's BIOS allows the BIOS to boot from a fakeraid
array, but once the OS has taken over, the driver performs all the RAID
functionality - akin to Linux software RAID (md), but at a lower level in
the stack.

> Thanks for any help
> Bobby

HTH,
Alex